System for CO2 capture with improved stripper performance

ABSTRACT

The present application relates to a system for removal of gaseous contaminants from a gas stream. The system includes an absorber for contacting the gas stream with a wash solution to form a used wash stream, a regenerator for regenerating the used wash solution, a reboiler and at least two heat exchangers in fluid communication with the absorber, regenerator and reboiler.

CROSS-REFERENCE

This application claims priority to, and is a divisional application of,U.S. application Ser. No. 12/732,961 filed Mar. 26, 2010, which in turnclaims priority to U.S. Provisional Patent Application Ser. No.61/164,944 filed Mar. 31, 2009, both of which are hereby incorporated byreference in their entireties.

TECHNICAL FIELD

The present application relates to methods and systems for removal ofgaseous contaminants from gas streams.

BACKGROUND

In conventional industrial technologies for gas purification,impurities, such as H₂S, CO₂ and/or COS are removed from a gas streamsuch as flue gas, natural gas, syngas or other gas streams by absorptionin a liquid solution, e.g. in a liquid solution comprising an aminecompound.

Used liquid solution is subsequently regenerated in a regenerator columnto release the impurities comprised in the solution, typically bycountercurrent contacting with steam. The steam needed for regenerationis typically produced by boiling the regenerated liquid solution in areboiler, located at the bottom at the regenerator column. In addition,reboiling may provide further release of impurities comprised in theliquid solution.

In conventional absorption-regeneration processes as described above,regenerated and reboiled liquid solution is typically re-used in anotherabsorption cycle. The reboiled solution may however have a temperatureas high as 100-150° C. To enable efficient absorption, liquid solutionsbased on amine compounds typically requires cooling before being passedto another round of absorption. Cooling has conventionally beenaccomplished by heat-exchange with used liquid solution from absorption.

The energy produced by the reboiler is not only required forregeneration, but also at other locations in an absorption-regenerationprocess. In general, the energy requirements of a conventional gaspurification process are of three types: binding energy, strippingenergy and sensible heat. Binding energy is required for breaking thechemical bond formed between the impurities and the liquid solution,whereas stripping energy is required for production of the steam neededfor releasing the impurities from the liquid solution. Sensible heat isin turn needed for heating of the liquid solution prior to regeneration.In conventional systems and processes, part of the produced energy maybe lost for example in the system coolers, which reduce the temperatureat specified locations in the system, e.g. the cooler located near theabsorber inlet for cooling return wash solution before feeding it to theabsorber. Moreover, energy may be lost in condensers located at the topof the absorber, regenerator etc, and in the form of water vapor exitingthe process, mostly at the top of the regenerator where water vapor ispresent in the purified CO₂ gas.

Thus, gas removal, and in particular regeneration, is an energyintensive process. Reduction of energy requirements at different partsof a gas purification process could potentially reduce the total energyrequired by the process.

U.S. Pat. No. 4,152,217 discloses an absorption-regeneration processwith reduced overall heat energy requirements. The process comprises asplit-flow arrangement in that the spent impurity-enriched solution,resulting from absorption, is split into two streams. The first streamis directed to the top of the regenerator column without being heated.The second stream is, after being heated by heat-exchange with the hotlean stream from the bottom of the regenerator column, fed to a lower,intermediate point in the regenerator column.

In WO09/112518, a process for removal of CO₂ is disclosed wherein theabsorbing liquid enriched in CO₂ is heated by heat-exchange prior tobeing fed to a regenerator column. Before being subjected toheat-exchange, the absorbing liquid is split into two streams. The firststream is heated by heat-exchange with regenerated liquid, and thesecond stream is heated by heat-exchange with stripping gas enriched inCO₂ from the top of the regenerator column.

EP 1 759 756 discloses a CO2 recovery process wherein a solution rich inCO₂ is regenerated in a regenerator tower comprising a heating member.The heating member heats the rich solution in the regenerator tower withsteam generated when regenerating the rich solution in the regeneratortower.

Although various improvements of conventional gas purificationtechnologies are known, there is an ever-existing desire to furtherimprove these technologies, in particular in respect of energyconsumption.

SUMMARY

The above drawbacks and deficiencies of the prior art are overcome oralleviated by a process for removal of gaseous contaminants from a gasstream comprising contacting the gas stream with a wash solution toremove gaseous contaminants from the gas stream by absorption into thewash solution; and regenerating the used wash solution to remove gaseouscontaminants from the used wash solution, to provide a regenerated washsolution and a gas comprising removed contaminants, wherein in a firstregeneration stage the gas comprising removed contaminants is cooled tominimize loss of water vapor from the regeneration step.

In this context, the terms “heated” and “cooled” are intended to referto relative temperatures of liquids, solutions or streams in gaspurification processes or systems. For example, a liquid may be referredto as a “heated” liquid following a heating step as compared to therelative temperature of the liquid prior to heating and/or, accordingly,as compared to the relative temperature of a liquid, solution or stream,at a corresponding location in a process lacking the heating step. Theterm “cooled” is correspondingly intended to refer to the relativetemperature of a liquid, solution or stream after a cooling step ascompared to the temperature of the liquid, solution or stream beforecooling or as compared to a liquid, solution or stream at acorresponding location in a process lacking the cooling step.

As used herein, “heat-exchanging” or “heat-exchange” imply a processstep wherein heat is deliberately transferred from one medium to anothermedium. Heat-exchange results in one medium leaving the process stepcooler than previously and one medium leaving the process step warmerthan previously, i.e. before the process step. Heat-exchange may bedirect, wherein the two media physically meet, or indirect, wherein themedia are separated, e.g. by a solid wall allowing heat transfer.

As used herein, “regeneration stage” does not mean an equilibrium stage,but a step in the regeneration process.

As outlined above, cooling here means cooled compared to the temperatureof a similar operation in a conventional process. Thus, cooling the gascomprising the contaminants removed from the used wash solution in afirst regeneration stage means providing a gas having a lowertemperature as compared to in a conventional process. By such cooling ina first regeneration stage, such as in an upper part of a regenerator,loss of water vapor from regeneration is minimized. In other words,water vapor is condensed before leaving the regeneration step and, as aconsequence, stripping energy is minimized. Cooling may however lead tore-absorption of a small amount of removed contaminants into the washsolution. As the used wash solution in the first regeneration stage isrich in contaminants, the extent of re-absorption will be low. However,the temperature of the gas comprising removed contaminants may beadapted in order to balance the extent of re-absorption and the extentof condensation.

In one embodiment, the used wash solution is cooled before being passedto regeneration. Thus, the used wash solution is cooled as compared tothe temperature of a used wash solution in a conventional process. As anexample, the used wash solution may have a temperature of below 115° C.In this way, the cooled used wash solution may provide the cooling ofthe gas comprising contaminants in the first regeneration stage.

In another aspect, there is provided a process for removal of gaseouscontaminants, such as CO₂, from a gas stream, comprising

a) contacting a gas stream comprising gaseous contaminants with a washsolution, to remove gaseous contaminants from the gas stream byabsorption into the wash solution;

b) passing used wash solution resulting from step a) to regeneration;

c) regenerating used wash solution by removal of gaseous contaminantsfrom the wash solution, to provide a regenerated wash solution and a gascomprising removed contaminants, wherein regeneration optionallycomprises a set of consecutive regeneration stages;

d) passing the regenerated wash solution to reboiling;

e) reboiling the regenerated wash solution for further removal ofgaseous contaminants from the wash solution, to provide a reboiled washsolution;

f) returning the reboiled wash solution to step a);

-   -   wherein during step f), the return wash solution is subjected to    -   heat-exchanging with regenerated wash solution of step d) in a        first heat-exchanging step, to heat the regenerated wash        solution; and    -   heat-exchanging with used wash solution of step b) in a second        heat-exchanging step, to heat the used wash solution.

Following reboiling of step e), the reboiled wash solution, referred toas the return wash solution, is returned to the absorption step, whereit once again is contacted with a gas stream containing gaseouscontaminants. During the return passage, the return wash solution issubjected to heat-exchanging in two consecutive heat-exchanging steps.First, transfer of heat is allowed for between the return wash solution,originating from reboiling, and the regenerated wash solution by theheat-exchanging of step d). Thus, this provides a cooled return washsolution and a heated regenerated wash solution. By this firstheat-exchange, heat energy provided to the process by reboiling is to agreater extent kept in the hot region of the process, i.e. the reboilingregion and the regeneration region closest to the reboiling region. Thismay reduce the amount of heating required in the reboiling step for theregenerated wash solution. In addition, the return wash solution leavesthe hot region cooler, which may reduce the cooling required beforecontacting the solution with a gas stream comprising gaseouscontaminants in the next process cycle.

In a second heat-exchanging step, the cooled return wash solution isheat-exchanged with used wash solution resulting from the absorptionstep. This allows for heat transfer between the used wash solution andthe return wash solution to cool the return wash solution prior to thenext absorption round of step a). The used wash solution resulting fromthe second heat-exchanging is heated, although to a less extent than ina conventional process. As a consequence, the less heated used washsolution may reduce the temperature for regeneration compared toconventional regeneration temperatures, In particular, the temperaturein the beginning of the regeneration, i.e. the temperature of the firstregeneration stages, typically in the regenerator overhead, may bereduced. This may help to reduce stripping energy losses duringregeneration, e.g. by minimizing the amount of water vapor escaping fromthe regeneration step.

According to examples as illustrated herein, there is provided aprocess, wherein in a first regeneration stage, the used wash solutioncools the gas comprising removed contaminants to minimize water vaporloss. Such cooling provides reduction of water vapor loss fromregeneration. This may in turn enable recycling of the stripping heatback to the regeneration and reboiling steps in the form of condensedwater. A larger degree of recycling of stripping heat provides forlarger energy recycling to the hotter region of the process, which mayreduce the overall reboiler duty. Altogether, the temperaturedifferential between the beginning and the end of the regeneration stepis increased as compared to conventional processes, which may allow forimproved stripping with limited water vapor loss.

According to examples as illustrated herein, there is provided a processcomprising a third heat-exchanging step between the first and secondheat-exchanging steps. In particular, such a process further compriseswithdrawing a first portion of used wash solution resulting from thesecond heat-exchanging step; and between the first and secondheat-exchanging steps, subjecting the return wash solution toheat-exchanging with the withdrawn first portion of used wash solutionin a third heat-exchanging step, to heat the used wash solution. Theportion withdrawn may for example constitute 10-90%, such as 50-90%,such as 75-90%, of the used wash solution resulting from the secondheat-exchanging step. It is understood that used wash solution may bedivided in any suitable number of portions.

In one example of a process for removal of gaseous contaminants, usedwash liquid remaining after withdrawal of the first portion is passed toregeneration. Passing a smaller portion of used wash solution toregeneration may lower the flow rate of the solution in regenerationresulting in the lowering of auxiliary energy consumption, i.e. energyconsumption associated with pumps, valves etc.

A third heat-exchanging step as described above may thus enable furthercooling of the return wash solution. This affects the secondheat-exchanging step, which, as a result, provides a less heated usedwash solution as compared to a conventional process. Consequently, theportion of used wash solution which is passed to regeneration, referredto as the second portion or the remaining portion, may further reducethe temperature in the regeneration step, e.g. in a first regenerationstage. In particular, the temperature of the gas mixture leavingregeneration may be further reduced which may further reduce loss ofwater vapor from the regeneration step. In this way, stripping energymay also be further reduced.

According to other examples of a process as illustrated herein, there isfurther provided a process comprising passing the first portion of usedwash solution resulting from the third heat-exchanging step toreboiling; and reboiling the first portion of used wash solution forremoval of gaseous contaminants from the used wash solution, to providea reboiled wash solution. The thus heated first portion of used washsolution is passed to reboiling, where gaseous contaminants and watervapor are released from the solution. The relatively higher temperatureof the wash solution entering the reboiling step enhances the removal ofgaseous contaminants from that reboiler area of the process.

During the passage to reboiling, the first portion of used wash solutionmay be combined with the regenerated wash solution of step d). Thus, thefirst portion of used wash solution may e.g. be mixed with theregenerated wash solution and subsequently subjected to heat-exchangingin the first heat-exchanging step. The first heat-exchanging step henceprovides a heated mixture comprising the first portion of used washsolution and the regenerated wash solution. The mixture is subsequentlysubjected to reboiling for removal of gaseous contaminants from themixture and for production of steam.

Alternatively, the first portion of used wash solution may, afterheating in the third heat-exchanging step as described above, be passedto regeneration where it may be regenerated for removal of gaseouscontaminants, to provide a regenerated wash solution. The first portionof used wash solution may for example be passed to a regeneration stagedownstream of the regeneration stage to which the second portion of usedwash solution is passed. In particular, the first portion of used washsolution may be regenerated in one or more regeneration stage(s)representing the lower and hotter part of regeneration.

Features mentioned in respect of the above aspects may also beapplicable to the aspects as described below.

According to other aspects as illustrated herein, there is provided agas purification system for removal of gaseous contaminants, such asCO₂, from a gas stream, comprising

an absorber for receiving a gas stream comprising gaseous contaminantsand contacting it with a wash solution;

a regenerator for regenerating used wash solution by releasing gaseouscontaminants from the wash solution;

a reboiler for reboiling regenerated wash solution to release gaseouscontaminants from the wash solution and for steam generation;

a first heat-exchanger for heat transfer between a reboiled washsolution and a regenerated wash solution, the heat-exchanger beingarranged for receiving reboiled wash solution from the reboiler and forreceiving regenerated wash solution from the regenerator; and

a second heat-exchanger for heat transfer between a reboiled washsolution from the first heat-exchanger and used wash solution from theabsorber, the heat-exchanger being arranged for receiving used washsolution from the absorber and for receiving reboiled wash solution fromthe first heat-exchanger;

wherein the absorber is arranged for receiving reboiled wash solutionfrom the second heat-exchanger; the reboiler is arranged for receivingregenerated wash solution from the first heat-exchanger; and theregenerator is arranged for receiving used wash solution from the secondheat-exchanger.

In the first heat-exchanger, heat transfer is provided for between areboiled wash solution, also denoted a return wash solution, and aregenerated wash solution. In the second heat-exchanger, the reboiledwash solution from the first heat-exchanger is heat-exchanged a secondtime. Thus, the second heat-exchanger provides heat transfer between thereboiled wash solution from the first heat-exchanger and used washsolution from the absorber. In the first heat-exchanger, the reboiledreturn wash solution gives up heat to the regenerated wash solution andin the second heat-exchanger the reboiled return wash solution gives upheat to the used wash solution. This enables recovery of heat from thehot reboiled wash solution, and at the same time recovery of heat fromthe vapor in the regenerator. Thus, heat is kept in the hot region ofthe system and the overall heat requirements of the system may bereduced.

Further, the regenerator receives used wash solution from the secondheat-exchanger. The wash solution is heated from heat-exchanging, butless heated compared to in conventional systems. This may in turn resultin a reduced regenerator temperature compared to the regeneratortemperature of a conventional system. The regenerator may for example bearranged for receiving used wash solution at an upper part of theregenerator, and thus in particular the temperature of the upper part ofthe regenerator may be reduced in order to control the temperature ofthe regenerator overhead.

In one example of a gas purification system, the system furthercomprises a third heat-exchanger for heat transfer between reboiled washsolution from the first heat-exchanger and a first portion of used washsolution from the second heat-exchanger, wherein the thirdheat-exchanger is arranged for receiving reboiled wash solution from thefirst heat-exchanger and for receiving a first portion of used washsolution from the second heat-exchanger. Such a system may for examplecomprise a flow splitter for dividing the used wash solution intoportions, such as two, three or more portions. The first portion isreceived by the third heat-exchanger whereas the remaining used washsolution may be received by the regenerator. By passing only a portionof the used wash solution to the regenerator, the total volume receivedby the regenerator is reduced, which in turn may reduce the flow rate inthe regenerator and, in addition, the regenerator size. This may howeverhave some impact on the reboiler sizing.

In a system comprising a third heat-exchanger as described above, thesecond heat-exchanger may be arranged for receiving reboiled washsolution from the third heat-exchanger; and the first heat-exchanger maybe arranged for receiving the first portion of used wash solution fromthe third heat-exchanger. Such a system may additionally comprise amixer for mixing the first portion of used wash solution from the thirdheat-exchanger with the regenerated wash solution, to provide a mixedsolution. This mixed solution may subsequently be directed to the firstheat-exchanger, which in this case is arranged to receive a mixedsolution from the mixer. After heat transfer in the first heat-exchangeras previously described, the mixture may be passed to reboiling in thereboiler for removal of gaseous contaminants and for production of steamto drive the regenerator.

In another example of a gas purification system, the regenerator isarranged for receiving the portion of used wash solution from the thirdheat-exchanger; and the second heat-exchanger is arranged for receivingreboiled wash solution from the third heat-exchanger. Here, theregenerator receives both the first portion of used wash solution aswell as the remaining used wash solution. In one example, the remainingwash solution, in one or more portions, is received at an upper part ofthe regenerator, whereas the first portion is received at a bottom partof the regenerator.

The above described and other features are exemplified by the followingfigures and the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the Figures, which are exemplary embodiments, andwherein the like elements are numbered alike:

FIG. 1 is a diagram generally depicting a conventional amine-basedsystem for removal of CO₂ from a gas stream.

FIG. 2 is a diagram generally depicting an example of a system forremoval of CO₂ from a gas stream as disclosed herein.

FIG. 3 is a diagram generally depicting an example of a system forremoval of CO₂ from a gas stream as disclosed herein.

FIG. 4 is a diagram generally depicting an example of a system forremoval of CO₂ from a gas stream as disclosed herein.

DETAILED DESCRIPTION

FIG. 1 illustrates a conventional system for removal of CO₂ from a gasstream. The system comprises an absorber (not shown), wherein a gasstream containing CO₂ is contacted, for example in a countercurrentmode, with a wash solution, such as an amine-based wash solution. In theabsorber, CO₂ from the gas stream is absorbed in the wash solution. Usedwash solution enriched in CO₂ leaves the absorber via line 101. TheCO₂-enriched wash solution is passed via a heat-exchanger 109 and line102 to a regenerator 103, wherein the used wash solution is stripped ofCO₂ by breaking the chemical bond between the CO₂ and the solution.Regenerated wash solution leaves the regenerator bottom via line 104.Removed CO₂ and water vapor leaves the process at the top of theregenerator via line 105. In addition, a condenser may be arranged atthe top of the regenerator to prevent water vapor from leaving theprocess.

Regenerated wash solution is passed to a reboiler 106 via line 104. Inthe reboiler, located at the bottom of the regenerator, the regeneratedwash solution is boiled to generate vapor 107 which is returned to theregenerator to drive the separation of CO₂ from wash solution. Inaddition, reboiling may provide for further CO₂ removal from theregenerated wash solution.

Following reboiling, the reboiled and thus heated wash solution is vialine 108 passed to a heat-exchanger 109 for heat-exchanging with theused wash solution from the absorber. Heat-exchanging allows for heattransfer between the solutions, resulting in a cooled reboiled washsolution and a heated used wash solution. The reboiled andheat-exchanged wash solution is thereafter passed to the next round ofabsorption in the absorber. Before being fed to the absorber, the washsolution 110 may be cooled to a temperature suitable for absorption.Accordingly, a cooler may be arranged near the absorber solvent inlet(not shown).

According to examples as illustrated herein, the gas stream comprisinggaseous contaminants may be a natural gas stream or a flue gas stream.In other examples of processes and systems as described herein, thegaseous contaminants may be acidic contaminants such as CO₂, H₂S etc.

A wash solution used for removal of gaseous contaminants may for examplebe an amine-based wash solution. Examples of amine-based wash solutionsinclude, but are not limited to, amine compounds such asmonoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine(MDEA), diisopropylamine (DIPA) and aminoethoxyethanol (diglycolamine)(DGA). The most commonly used amines compounds in industrial plants arethe alkanolamines MEA, DEA, MDEA and some blends of conventional amineswith promoters, inhibitors etc. However, it is understood that thesystems and processes as herein disclosed may be applied to any solutioninvolved in a process with an absorption/regeneration scheme.

FIG. 2 is a schematic representation of a system for removal of CO₂ froma gas stream as described herein. The system comprises an absorber (notshown) for receiving a gas stream comprising CO₂ and contacting it witha wash solution. CO₂ is removed from the gas stream by absorption intothe wash solution. Used wash solution is subsequently passed via line201 to a heat-exchanger 209.

The heat-exchanger used for heat transfer between two solutions in asystem and a process as disclosed herein may for example be acountercurrent heat-exchanger. Examples of heat-exchangers include, butare not limited to, shell-and-tube heat-exchangers, and plate and frameheat-exchangers.

From the heat-exchanger, used wash solution is passed via line 202 to aregenerator 203. As described above, the used wash solution is strippedof CO₂ in the regenerator and removed CO₂ leaves the regenerator vialine 205. The regenerator may for example be a column, such as a packedbed column or a column comprising trays.

Regenerated wash solution leaves the regenerator bottom via line 204 andis passed to reboiling via a heat-exchanger 211. In the heat-exchanger,the regenerated wash solution is heat-exchanged with reboiled washsolution from the reboiler 206. Thus, heat-exchanging provides a cooledreboiled wash solution 213 as compared to the temperature of washsolution 208. In addition, heat-exchanging in heat-exchanger 211provides a heated regenerated wash solution 212, which is passed toreboiling in the reboiler 206, as compared to the temperature of theregenerated wash solution 204 coming from the regenerator. In this way,heat energy is kept in the hot region of the system, i.e. the reboilerand the lower part of the regenerator.

After heat-exchange in heat-exchanger 211, the regenerated wash solutionis passed to reboiling via line 212. In the reboiler 206, theregenerated wash solution is boiled to produce vapor 207 to drive theseparation process in the regenerator 203. It is to be understood thatany type of reboiler may be used in the herein disclosed system.Examples of reboilers include, but are no limited to,horizontal/vertical thermosyphon type reboilers, and kettle typereboilers.

Following the first heat-exchanging, the reboiled wash solution isreturned to the absorber via a second heat-exchanger 209. In similaritywith what is described above, the return wash solution 213 isheat-exchanged with the used wash solution 201 in the heat-exchanger.The two consecutive heat-exchangers lower the temperature of the returnwash solution to be recycled to the absorber as compared to the washsolution 110 recycled to the absorber in FIG. 1. The lower temperatureof the return wash solution 213 may provide for more efficientheat-exchanging in heat-exchanger 209, as compared to heat-exchanging inthe corresponding heat-exchanger 109 in FIG. 1. If, for example, thereturn wash solution is recycled to the next round of absorption bypumping, the cooled return wash solution may reduce the risk ofcavitation during pumping.

More efficient heat-exchanging in the second heat-exchanger results in aused wash solution 202 with lower temperature as compared to the usedwash solution 102. Lowered temperature of used wash solution 202 may, asa consequence, reduce the temperature at the regenerator 203. As aresult of this, the temperature downstream of the heat-exchanger 209will be lower compared to the temperature downstream the heat-exchanger109, which in turn may reduce the cooling duty in a cooler arranged nearthe absorber solvent inlet.

FIG. 3 is a schematic representation of a system for removal of CO₂ froma gas stream as described herein. Components that are the same as thosein FIG. 2 are assigned with the corresponding reference numerals, andexplanation thereof is omitted. Reference numerals where the two lastfigures (e.g. 2##, 3##) are the same represent the same components.

During the passage to regeneration, the wash solution is here divided intwo portions in flow splitter 314. A first portion of the used washsolution 315 is passed to a third heat-exchanger 316, located betweenthe first 311 and the second heat-exchanger 309. In the thirdheat-exchanger, the return wash solution 313 is subjected toheat-exchanging with the first portion of used wash solution 315, tocool the return wash solution. The first portion of used wash solutionmay amount to 10-90%, such as 50-90%, or such as 75-90% of the volume ofused wash solution resulting from the second heat-exchanger.

Following heat-exchanging in the third heat-exchanger, the return washsolution is, via line 317, passed to the second heat-exchanger 309 forheat-exchanging with used wash solution 301 from the absorber. Thus, thereturn wash solution is here subjected to three consecutiveheat-exchanging steps, in which heat is recovered from the return washsolution. In addition, the temperature of the used wash solution 302 isaffected, coming out of the second heat-exchanger cooler than in systemslacking the features described herein.

The second portion of used wash solution 318 is passed to regeneration,where it, due to its reduced temperature compared to conventionalsystems, reduces the temperature of the gas mixture 305 leaving theregenerator 303. As a consequence, the amount of water vapor leaving theregenerator may be decreased, since water condenses and drains back tothe bottom of the regenerator 303. In this way, the stripping heat maybe recycled to the regenerator 303 and reboiler 306 in the form ofcondensed water. When a larger amount of energy is recycled to thehotter part of the system (regenerator and reboiler), the overallreboiler duty may be reduced.

Furthermore, the flow rate of the second portion of wash solutionentering the regenerator may be reduced depending on the size of thesecond portion, i.e. split ratio of the solution in 314. If a minorportion is passed to regeneration, the regenerator size may besignificantly decreased. In this case the major portion of used washsolution is passed to reboiling, via heat-exchanging, and thus thereboiler size may increase due to increased flow rate through thereboiler

Regenerated wash solution stripped of CO₂ leaves the regenerator vialine 304. In one example of a system as illustrated herein, theregenerated wash solution is combined with the first portion of usedwash solution 319 from the third heat-exchanger 316. Combination may forexample take place in a mixer 320, and the resulting mixture maysubsequently be passed to the first heat-exchanger 311 via line 321. Inthe first heat-exchanger, the mixture is subjected to heat-exchangingwith the reboiled wash solution 308, whereafter the heated mixture 312itself is fed to the reboiler 306 for reboiling. In the reboiler,further CO₂ removal may take place. In addition, vapor 307 is producedfor driving the regeneration.

FIG. 4 is a schematic representation of a system for removal of CO₂ froma gas stream as described herein. Once again, components that are thesame as those as previously described are assigned with the samereference numerals.

In the third heat-exchanger 416, the first portion of used wash solutionis heat-exchanged with the return wash solution from the firstheat-exchanger 411. Heat-exchanging in the third heat-exchanger enablesheat recovery from the return wash solution 417, which comes out of theheat-exchanger cooler than before heat-exchanging.

In the third heat-exchanger 416, heat is recovered in the first portionof used wash solution 422. This portion of wash solution is fed to theregenerator 403. The heated portion of used wash solution may be fed tothe regenerator at a middle or bottom part of the regenerator. If theregenerator for example is a column comprising a number of trays, theportion of used wash solution may e.g. be fed to the lower traysrepresenting the hottest region of the system. It is understood that asuitable introduction level of the regenerator, or suitable regenerationstage, may depend on the split ratio of the used wash solution. Thefirst portion, which in this example is passed to the regenerator, maycomprise the major part of the used solution, such as 75-90% of the usedwash solution.

While the invention has been described with reference to variousexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

EXAMPLE

In the following example, energy requirements were calculated forsimulations of different CO₂ removal processes as disclosed herein andcompared to energy requirements for a simulated conventional CO₂ removalprocess. For all simulations, the following conditions were used:

Wash solution MDEA based

Regenerator pressure 50 psi

Temperature approach heat-exchanger 5° C.

(Temperature difference between the cold stream inlet and the hot streamoutlet)

Lean loading 0.088 mol/mol

(Absorber inlet/reboiler outlet)

Rich loading 0.3 mol/mol

(Absorber outlet)

Energy Requirements for a Conventional Process

In this example, a conventional process was simulated comprising thegeneral steps of absorbing CO₂ into a MDEA based solution, regeneratingthe CO₂ enriched solution (rich solution), reboiling the regeneratedsolution and cooling of the reboiled solution (lean solution) in aheat-exchanger before recycling to the absorption step. Such a processis schematically represented by FIG. 1.

Conventional processes for CO₂ removal have an inherent efficiency ofaround 3-4 GJ/tonne CO₂ captured. The energy required by the process isprovided mostly by a reboiler, located at the bottom of the regenerator.The distribution of the energy requirements for the conventional processwas as presented in the table below.

TABLE 1 Process energy requirements for a conventional process % ofreboiler duty binding energy 40-50 sensible heat 10-30 stripping energy30-40

For this conventional process, used MDEA based solution passed to theregenerator held a temperature of approximately 115° C. The temperatureof the wash solution in turn influenced the temperature at the top ofthe regenerator.

Energy Requirements for a First Example Process

In this first example, a process for CO₂ removal similar to the oneschematically depicted in FIG. 2 was simulated. The simulated processthus comprised a first and a second heat-exchanger for recovery of heatfrom the hot reboiled return solution. This scheme with twoheat-exchangers also affected the temperature of the rich MDEA basedsolution reaching the top of the regenerator. The temperature of therich solution at the regenerator top was here lower as compared to therich solution at the corresponding location in the conventional processdescribed above.

Compared to the conventional process as described above, the reboilerduty was now reduced to 2.18 GJ/tonne CO₂ captured. The distribution ofenergy requirements was as presented below.

TABLE 2 Process energy requirements for a first example process % ofreboiler duty binding energy ca 70 sensible heat ca 10 stripping energyca 20

In this example, reboiler duty was thus reduced compared to theconventional process described above, and the energy requirements wereredistributed. The binding energy, i.e. the energy required for breakingthe chemical bond between CO₂ and the wash solution here required mostof the reboiler duty.

Energy Requirements for a Second Example Process

In the second example, a process similar to the one schematicallyillustrated in FIG. 3 was simulated. Compared to the first exampleprocess, the second example process also comprised a thirdheat-exchanger between the first and second heat-exchangers. This thirdheat-exchanger allowed for even further heat recovery from the returnlean solution. Thus, the return lean solution subjected toheat-exchanging in the second heat-exchanger was thus cooler than thecorresponding return lean solution of the above described conventionalprocess. In consequence, the heated rich solution reaching theregenerator top was cooler as compared to the corresponding solution ofthe conventional process.

The process of the second example further comprised a divider fordividing rich solution into two portions. Approximately 90% of the richsolution was passed to the third heat-exchanger as described above andless than 10% was fed to the regenerator. The portion of rich solutionthat was heat-exchanged in the third heat-exchanger, was further,following mixing with the regenerated wash solution and heat-exchangingin the first heat-exchanger, fed to the reboiler.

Compared to the conventional process as described above, the reboilerduty of the process was reduced to 2.11 GJ/tonne CO₂ and the energyrequirements were as shown below.

TABLE 3 Process energy requirements for a second example process % ofreboiler duty binding energy ca 70 sensible heat ca 15 stripping energyca 25

At the top of the regenerator, the cool wash solution with low flow ratewas contacted with the gas mixture rich in CO₂. This might lead to aslight re-absorption of CO₂ at the top of the regenerator. However,since the used wash solution is already rich in absorbed CO₂ and sincethe flow rate of the solution is lowered, the amount of re-absorbed CO₂may be limited. Thus, the process still allows for significant overallremoval of CO₂. Furthermore, the re-absorbed CO₂ releases its bindingenergy (exothermic reaction) and this energy may be captured in the formof sensible heat contained in the solution flowing down the regeneratorand the reboiler. Thus, this energy is not lost to the ambient, whichmay further reduce the reboiler needs.

Energy Requirements for a Third Example Process

In the third example, a process similar to the one schematicallyillustrated in FIG. 4 was simulated. The third example process differedfrom that in the second example in that the portion of rich solutionsubjected to heat-exchanging in the third heat-exchanger was fed to thebottom of the regenerator. Here, less than 10% of the rich solution wasdirected to the top of the regenerator while the remaining 90% was,following heat-exchanging, fed to a lower part of the regenerator.

Compared to the conventional process, the reboiler duty of this processwas reduced to 1.82 GJ/tonne CO₂ and the energy requirements were:

TABLE 4 Process energy requirements for a third example process % ofreboiler duty binding energy ca 80 sensible heat ca 15 stripping energy<5

This last example process showed that the binding energy representedaround 80% of the total reboiler duty. With such configuration, thetemperature at the top of the regenerator was drastically reduced toapproximately 57° C., which is approximately 21° C. above thetemperature of the rich solution coming from the absorber, which furtherreduced the stripping energy requirements.

Although the simulations were performed for a high pressure regenerator,similar results are expected for processes with a regenerator at lowerpressure. Also, the temperature approach could probably be increased toreasonable values without greatly influencing the above energydistribution patterns.

The invention claimed is:
 1. A gas purification system for removal ofgaseous contaminants from a gas stream, comprising: an absorber forcontacting a gas stream comprising gaseous contaminants with a washsolution to form a used wash solution; a regenerator for generating aregenerated wash solution from the used wash solution by releasinggaseous contaminants from the used wash solution; a reboiler forreboiling the regenerated wash solution to release gaseous contaminantsto form a reboiled wash solution and steam; a first heat-exchanger forheat transfer between the reboiled wash solution and the regeneratedwash solution; a second heat-exchanger for heat transfer between thereboiled wash solution from the first heat-exchanger and the used washsolution from the absorber, wherein the absorber is arranged forreceiving the reboiled wash solution from the second heat-exchanger; thereboiler is arranged for receiving the regenerated wash solution fromthe first heat-exchanger; and the regenerator is arranged for receivingthe used wash solution from the second heat-exchanger; and a thirdheat-exchanger for heat transfer between the reboiled wash solution fromthe first heat-exchanger and a first portion of the used wash solutionfrom the second heat-exchanger; and wherein the third heat-exchanger isarranged for receiving reboiled wash solution from the firstheat-exchanger and for receiving a first portion of used wash solutionfrom the second heat-exchanger.
 2. The gas purification system accordingto claim 1, wherein the regenerator is arranged for receiving used washsolution at an upper part of the regenerator.
 3. The gas purificationsystem according to claim 1 wherein the first portion of used washsolution is 10-90% of the used wash solution resulting from the secondheat-exchanger.
 4. The gas purification system according to claim 1,wherein the second heat-exchanger is arranged for receiving reboiledwash solution from the third heat-exchanger; and the firstheat-exchanger is arranged for receiving the first portion of used washsolution from the third heat-exchanger.
 5. The gas purification systemaccording to claim 1, wherein the regenerator is arranged for receivingthe portion of used wash solution from the third heat-exchanger; and thesecond heat-exchanger is arranged for receiving reboiled wash solutionfrom the third heat-exchanger.
 6. The gas purification system accordingto claim 5, wherein the regenerator is arranged for receiving theportion of used wash solution resulting from the third heat-exchanger ata bottom part of the regenerator.